It is narrated from Sayyiduna Thawban (may Allah be pleased with him) that he said: When you see that black banners have come out from the direction of Khurasan, then be sure to reach them, even if you have to drag yourselves (on your knees), for among them will be the caliph of Allah, the Mahdi. This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it.